Eosinophilia

Eosinophilia is an increase in the number of eosinophils in the peripheral blood of more than 450/μl. There are many reasons for an increase in the number of eosinophils, but allergic reactions or parasitic infections are most common. Diagnosis involves selective testing aimed at a clinically suspected cause. Treatment is aimed at eliminating the underlying disease.

Eosinophilia has features of an immune response: an agent such as Trichinella spiralis promotes the development of a primary reaction with a relatively low level of eosinophils, the reappearance of the agent leads to an increase in the level of eosinophils or a secondary eosinophilic response.

Factors that decrease eosinophil numbers include beta blockers, glucocorticoids, stress, and occasionally bacterial or viral infections. Several compounds released by mast cells induce IgE-mediated eosinophil production, such as eosinophil chemotactic factor of anaphylaxis, leukotriene B4, complement complex (C5-C6-C7), and histamine (higher than normal concentrations).

Eosinophilia may be primary (idiopathic) or secondary to numerous diseases. In the United States, the most common causes of eosinophilia are allergic and atopic diseases, with respiratory and skin diseases being the most common. Almost all parasitic tissue invasions can cause eosinophilia, but protozoan and noninvasive metazoan invasions generally do not result in elevated eosinophil levels.

Neoplastic diseases, such as Hodgkin lymphoma, may cause significant eosinophilia, which is unusual in non-Hodgkin lymphoma, chronic myelogenous leukemia, and acute lymphoblastic leukemia. Among solid tumors, ovarian cancer is the most common cause of eosinophilia. Hypereosinophilic syndrome with pulmonary involvement includes a spectrum of clinical manifestations characterized by peripheral eosinophilia and eosinophilic pulmonary infiltrates, but the etiology is usually unknown. Patients with eosinophilic drug reactions may be asymptomatic or may present with a variety of syndromes, including interstitial nephritis, serum sickness, cholestatic jaundice, hypersensitivity vasculitis, and immunoblastic lymphadenopathy. Several hundred patients have been reported to have eosinophilic myalgia syndrome after taking L-tryptophan for sedation or psychotropic therapy. This syndrome is probably not due to L-tryptophan itself but to contamination. Symptoms (severe muscle pain, tendosynovitis, muscle swelling, skin rash) have lasted from weeks to months and have resulted in deaths.

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Main causes of secondary eosinophilia

Reasons	Examples
Allergic or atopic diseases	Asthma, allergic rhinitis, allergic bronchopulmonary aspergillosis, occupational lung disease, urticaria, eczema, atopic dermatitis, milk protein allergy, angioedema with eosinophilia, drug reaction
Parasitic infections (especially multicellular with tissue invasion)	Trichinellosis, visceral larva wandering syndrome, trichiuriasis, ascariasis, strongyloidiasis, cysticercosis (Taenia solium), echinococcosis, filariasis, schistosomiasis, nematodiasis, Pneumocystis jiroveci (former P. carinii)
Non-parasitic infections	Aspergillosis, brucellosis, cat scratch disease, infectious lymphocytosis, chlamydial pneumonia of infants, acute coccidioidomycosis, infectious mononucleosis, mycobacterial diseases, scarlet fever
Tumors	Cancer and sarcomas (lung, pancreas, colon, cervix, ovaries), Hodgkin's lymphoma, non-Hodgkin's lymphomas, immunoblastic lymphadenopathy
Myeloproliferative diseases	Chronic myelogenous leukemia
Pulmonary infiltration syndromes with eosinophilia	Simple pulmonary eosinophilia (Loeffler syndrome), chronic eosinophilic pneumonia, tropical pulmonary eosinophilia, allergic bronchopulmonary aspergillosis, Churg-Strauss syndrome
Skin diseases	Exfoliative dermatitis, dermatitis herpetiformis, psoriasis, pemphigus
Connective tissue diseases or granulomatous diseases (especially involving the lungs)	Polyarthritis nodosa, rheumatoid arthritis, sarcoidosis, inflammatory bowel disease, SLE, scleroderma, eosinophilic fasciitis
Immune diseases	Graft-versus-host disease, congenital immunodeficiency syndrome (eg, IgA deficiency, hyper IgA syndrome, Wiskott-Aldrich syndrome)
Endocrine diseases	Adrenal hypofunction
Different	Cirrhosis, radiation therapy, peritoneal dialysis, familial eosinophilia, L-tryptophan use

Diagnosis and treatment of eosinophilia

When eosinophilia is present in the peripheral blood, an absolute eosinophil count is often unnecessary. A complete history, particularly of travel, allergies, and medication use, should be obtained, followed by an examination. Specific diagnostic tests should be determined based on the physical examination and may include chest radiography, urinalysis, liver and kidney function tests, and serologic tests for parasites and connective tissue disorders. Stool examination for parasites and ova is necessary, although a negative result does not exclude the absence of a parasitic infection (eg, trichinosis requires muscle biopsy; visceral larva migrans and filarial infections require other tissue biopsies; duodenal aspirate is needed to exclude specific parasites such as Strongyloides sp). Elevated serum vitamin B12 _or low leukocyte alkaline phosphatase or abnormal peripheral blood smear suggest a myeloproliferative disorder, requiring bone marrow aspirate and biopsy with cytogenetic analysis.

If the cause of eosinophilia is not found, the patient is at risk of complications. A test with a short administration of small doses of glucocorticoids will demonstrate a decrease in the number of eosinophils if eosinophilia is secondary (for example, allergy or parasitic infection), and will not have an effect in oncological diseases. Such a test is indicated for persistent or progressive eosinophilia and the absence of an apparent cause.

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